A. Technical Field
The present invention relates to a water-absorbent resin composition. Specifically, the invention relates to a water-absorbent resin composition wherein the water-absorbent resin composition is excellent in the urine resistance and the light resistance, and becomes little colored. Furthermore, the invention relates to a water-absorbent resin composition which is excellent also in the deodorizability.
B. Background Art
In recent years, water-absorbent resins are widely used as components of sanitary materials, such as disposable diapers, sanitary napkins and incontinent pads, for the purpose of causing the water-absorbent resins to absorb body liquids such as urine and menstrual blood.
As to the above water-absorbent resins, the following are known as their examples: partially-neutralized and crosslinked poly(acrylic acids); hydrolyzed graft polymers of starch-acrylonitrile; neutralized graft polymers of starch-acrylic acid; saponified copolymers of vinyl acetate-acrylic acid ester; crosslinked polymers of carboxymethyl cellulose; hydrolyzed copolymers of acrylonitrile or acrylamide, or crosslinked polymers of these hydrolyzed copolymers; crosslinked polymers of cationic monomers; crosslinked copolymers of isobutylene-maleic acid; and crosslinked copolymers of 2-acrylamido-2-methylpropanesulfonic acid-acrylic acid.
There is a problem which has hitherto been pointed out. The problem is that: in the case where the water-absorbent resin is used as an absorbent structure for disposable diapers, the water-absorbent resin deteriorates with the passage of time, so that its liquid permeability or gel strength becomes low, resulting in urine leakage from the disposable diapers. Such a deterioration of the water-absorbent resin due to urine is considered to be caused by a very small amount of metal ion and by L-ascorbic acid which is contained in urine. Thus, there is a proposed art in which a so-called chelating agent is added to the water-absorbent resin to scavenge the metal ion, thereby enhancing the urine resistance of the water-absorbent resin. Known examples of such a chelating agent include aminoacetic cheating agents such as sodium diethylenetriaminepentaacetate and sodium triethylenetetraminehexaacetate (JP-A-246674/1999, EP 940148A).
From structures of the aminoacetic chelating agents, it is expected that the aminoacetic chelating agents are able to enhance not only the urine resistance, but also the light resistance of the water-absorbent resin. However, prior arts enhance the urine resistance only and, as to the light resistance, provide the same results as those from no addition of the aminoacetic chelating agents.
On the other hand, the water-absorbent resin is desired to become little colored with the passage of time from a point of view that the water-absorbent resin is mainly used for sanitary materials (EP 942014). In addition, it is also desired to reduce the offensive odor which is emitted, for example, when the water-absorbent resin absorbs urine (U.S. Pat. No. 5,078,992, JP-A-085082/1997).
A. Objects of the Invention
A first object of the present invention is to provide a water-absorbent resin composition which contains an aminoacetic chelating agent and is excellent in the urine resistance, wherein the water-absorbent resin composition is excellent further in the light resistance. A second object of the present invention is to provide a water-absorbent resin composition which becomes little colored, and further, little smells bad when used.
B. Disclosure of the Invention
As a result of the present inventors"" diligent study to solve the above problems, it has been found that a cause of the non-enhancement of the light resistance in spite of the addition of the aminoacetic chelating agent in prior arts is nitrilotriacetic acid and/or its salt (hereinafter, for simplification, both might be generically referred to as xe2x80x9cnitrilotriacetic acid (salt)xe2x80x9d) which are contained as impurities in a concentration of not lower than several weight percent (usually, about 5 to about 15 weight %) in the aminoacetic chelating agent. The nitrilotriacetic acid (salt) is also a substance which gives anxiety that it might be carcinogenic (rank 2B in IARC). Therefore, its content must be reduced as much as possible also from a viewpoint of safety, considering that the water-absorbent resin composition is used for things contacting human bodies such as disposable diapers.
The aminoacetic chelating agent is generally produced by a process including the steps of causing ammonia to react with ethylene dichloride or aziridine to form a product such as diethylenetriamine or triethylenetetramine, and then causing this product to react with chloroacetic acid and an alkali agent such as sodium hydroxide. However, it is considered that unreacted ammonia remains in the above product such as diethylenetriamine or triethylenetetramine, and therefore reacts with chloroacetic acid to form the nitrilotriacetic acid (salt) as impurities.
From the results verified by the present inventors"" experiments, it has been found that the total content of the nitrilotriacetic acid (salt) in the water-absorbent resin composition needs to be reduced to not more than 1 ppm, preferably not more than 0.5 ppm, more preferably not more than 0.1 ppm (herein, unless otherwise noted, xe2x80x9cppmxe2x80x9d is by weight), of the water-absorbent resin composition from a point of view of enhancing the light resistance, and further that if the total content of the nitrilotriacetic acid (salt) in the water-absorbent resin composition is reduced to such a degree, the coloring of the water-absorbent resin composition is also little. However, if an attempt is made to reduce the total content of the nitrilotriacetic acid (salt) in the water-absorbent resin composition by simply reducing the mixing ratio of the aminoacetic chelating agent, the light resistance may be enhanced, but on the other hand the urine resistance is extremely deteriorated, that is to say, it becomes difficult to keep or enhance the urine resistance by adding the chelating agent. From the results verified by the present inventors"" experiments, it has been found that the mixing ratio of the aminoacetic chelating agent in the water-absorbent resin composition needs to be restricted to not less than 10 ppm, preferably not less than 20 ppm, of the water-absorbent resin from a point of view of keeping or enhancing the urine resistance.
From the above, it has been found that a water-absorbent resin composition which is excellent not only in the urine resistance, but also in the light resistance, and further, becomes little colored is obtained if the mixing ratio of the aminoacetic chelating agent in the water-absorbent resin composition is restricted to not less than 10 ppm of the water-absorbent resin and further if the total content of the nitrilotriacetic acid (salt) in the water-absorbent resin composition is restricted to not more than 1 ppm, preferably not more than 0.5 ppm, more preferably not more than 0.1 ppm, of the water-absorbent resin composition. Thus, the present invention has been completed.
The present inventors further studied diligently about a process for reducing the total content of the nitrilotriacetic acid (salt) in the aminoacetic chelating agent in order to facilitate the above execution, namely, the execution of the restrictions of the mixing ratio of the aminoacetic chelating agent in the water-absorbent resin composition to not less than 10 ppm of the water-absorbent resin and further the total content of the nitrilotriacetic acid (salt) in the water-absorbent resin composition to not more than 1 ppm of the water-absorbent resin composition. As a result, it has been found that when both the aminoacetic chelating agent and the nitrilotriacetic acid are in salt form (xe2x80x94COOX (X: e.g. alkaline metal)), both have high water solubility and are therefore difficult to separate from each other, but that when both are in acid form (xe2x80x94COOH), the aminoacetic chelating agent has low water solubility, while the nitrilotriacetic acid has high water solubility. Thus, the inventors have completed the present invention by finding that an aminoacetic chelating agent having an extremely reduced total content of the nitrilotriacetic acid (salt) for the water-absorbent resin is obtained if the above difference in water solubility is utilized to carry out a process including the steps of: adjusting pH of an aqueous solution of an aminoacetic chelating agent containing the nitrilotriacetic acid (salt) into the range of 1 to 3 to deposit a precipitate (the acid form of aminoacetic chelating agent); recovering the deposited precipitate by its separation; and washing and then drying the recovered precipitate, if necessary.
Thus, to solve the aforementioned problems, the present invention provides the following:
(1) A water-absorbent resin composition, which comprises a water-absorbent resin and an aminoacetic chelating agent, wherein the mixing ratio of the aminoacetic chelating agent in the water-absorbent resin composition is not less than 10 ppm of the water-absorbent resin, and wherein the total content of nitrilotriacetic acid and its salt in the water-absorbent resin composition is not more than 1 ppm of the water-absorbent resin composition.
(2) A water-absorbent resin composition, which comprises a water-absorbent resin and an aminoacetic chelating agent, wherein the total content of nitrilotriacetic acid and its salt relative to the aminoacetic chelating agent in the water-absorbent resin composition is not more than 1,000 ppm.
(3) A water-absorbent resin composition according to (1) or (2) above, which further comprises a water-soluble deodorant.
(4) A water-absorbent resin composition according to any one of (1) to (3) above, which exhibits an absorption capacity of not less than 30 g/g without load, an eluted deteriorated extractable content of not more than 20 weight %, an absorption capacity of not less than 28 g/g under a load of 1.9 kPa, and a yellowness index of not more than 26.
(5) A water-absorbent resin composition, which comprises a water-absorbent resin and a water-soluble deodorant and exhibits an absorption capacity of not less than 30 g/g without load, an eluted deteriorated extractable content of not more than 20 weight %, an absorption capacity of not less than 28 g/g under a load of 1.9 kPa, and an offensive-odor reduction ratio of not less than 40%.